A number of portable electronic devices may have a so-called split design where two or several parts of the device may be folded on each other or detached from each other. An example of such a device may be a portable telephone apparatus, a Personal Digital Assistant (PDA) apparatus or a laptop computer having for example two halves, a display half and a keyboard half. One example of such a portable device may be the Nokia Communicator. In such a split design device, the different parts need to communicate with each other. Another example of a split design device may be a telephone with detachable accessories, for example with a camera module, or a video camcorder.
Conventionally, the communication connection between the different parts of a split design devices is established by means of cables. For example in the split design telephone, information to be displayed on the display is typically conveyed from the keyboard via a flat cable, which may be a parallel data bus. The flat cable may be connected to one flat cable connector on a circuit board of the display side and to another flat cable connector on a circuit board of the keyboard side. Flat cable may be comprised of a plurality of metal wires to carry electrical current. These metal wires may be individually surrounded by a protective jacket. Bad connections may occur to the flat cable connectors over time. Furthermore, sharp objects and corrosive agents can accidentally break the flat cable, thereby interrupting the communication between the display and the keyboard. Moreover, a flat cable may be bulky as its width depends on the number of wires.
In U.S. Pat. No. 6,470,132, an alternative design for the cable connection is disclosed for a portable electronic device of the type described above. U.S. Pat. No. 6,470,132 describes an optical hinge, which serves as a light guide to convey communication data in optical signals between two circuit boards having opto-electronic devices for transmitting and receiving optical signals. The optical hinge of U.S. Pat. No. 6,470,132 may eliminate the above reliability problems associated with the flat cable and the flat cable connectors. The optical hinge of U.S. Pat. No. 6,470,132 may also provide a connection requiring less space than a cable connection as no mechanical connectors are needed.
In U.S. Pat. No. 5,914,976, vertical cavity surface emitting laser (VCSEL)-based multi-wavelength transmitter and receiver modules for serial and parallel optical links are disclosed. The module includes one or more VCSEL transmitters and photodetectors coincidentally aligned along a common central longitudinal axis. The incoming received light and transmitted light are at different wavelengths and are coupled out of and into the same optical fiber or free-space optical link. The VCSEL transmitter includes a pair of mirror stacks and an active region interposed between the pair of mirror stacks. The mirror stacks are each a system of alternating layers of gallium arsenide (GaAs) and aluminium gallium arsenide (AlGaAs) which forms a distributed Bragg reflector. At least one of the mirror stacks is wafer fused to the active region. One of the mirror stacks is below 0.0001 percent transmissive to the transmitted light. The VCSEL transmitter can be optically pumped by a pump VCSEL, wherein the pump laser radiation is for example an 850 nm pump VCSEL.
In U.S. Pat. No. 6,373,046, a small sized optical transmission unit suitable for non-simultaneous transmission and reception is disclosed. The transmission unit, which is smaller than previously known solutions, is made by mounting the preamplifier circuit, the receiver and the transmitter at least partially on top of each other instead of placing them side by side. The transmitter and the receiver use one and the same optical element. In U.S. Pat. No. 6,373,046, the transmitter may advantageously be a light emitting diode (LED) and the receiver a PIN diode.
However, optical interconnections have not been considered very practical in mobile terminals. Optical connections for data transmission purposes need typically be high-speed connections to provide sufficient data transmission efficiency. The efficiency of the opto-electronic transformers, such as LEDs, lasers, photo diodes, is rather low compared to electrical interconnections. Furthermore, in portable split design electronic devices, it is often required that the receiver part remains permanently awake or is regularly woken up to observe any signals coming in, for example if the keypad is being pressed. High-speed optical connections consume therefore rather a lot of power. However, low power consumption is a crucial factor for mobile terminals, such as portable telephones or PDAs.
The receiver side of an optical interconnection may be switched off to a sleep-mode for power saving purposes. There is, however, no easy way of quickly waking up the interconnection at a specific time, since the receiver cannot receive any signals in the sleep-mode.
The problem can be explained using a PDA device, such as the Nokia 9210 Communicator phone, as an example device. In such a PDA device, there may be a phone function having a small display and possibly a keypad and a PDA function having a larger display and a keyboard. The PDA display and phone display and the PDA keyboard are situated in separate parts communicating with each other. The connection between the separate parts is typically a high-speed data interconnection in order to satisfy the needs of the PDA display. This connection may conventionally be secured by a flex cable as explained above. A serial high-speed optical interconnection might be used to replace the relatively fragile and large flex cable. An optical connection would provide new mechanical design possibilities as it allows high freedom of movement of the separated parts communicating with each other. Using optical connection instead of a flex cable could also reduce the manufacturing time, as less manual work may be needed. The serial optical interconnection may, however, dissipate rather too much power at high data rates. However, when the Communicator's lid is closed, the PDA display may be switched off and no high-speed interconnection is needed anymore. Some interconnection is needed anyhow, since the keyboard functions and the small phone display are still active.
A solution could be to wake-up the high-speed optical receiver in the display part each once in a while for a short duration and check if a long-duration wake-up signal is received. In that case, the duration of the transmitter-initiated wake-up signal and the checking of the receiver, whether a wake-up signal is present, have to be adjusted carefully. The transmitter can switch to high-speed mode only after a worst-case time, which may be calculated from the receivers wake-up-checking periodicity plus the time needed for the receiver to wake up, since the transmitter side does not know when the receiver side wakes up. This makes the wake-up relatively slow and consumes power itself, but on the other hand requires relatively few components.
However, there is a need for an improved communication link for communicating data between a first opto-electronic equipment and a second opto-electronic equipment. It might be desirable to provide an optical high-speed connection with reduced power consumption. In certain cases, it may be advantageous to provide an optical high-speed connection, which is capable of being switched off when not in use and of becoming fully active in a short period of time.